Printers for printing desired information such as text and images on a sheet-like print medium such as paper or film are in wide use as information printing devices in word processors, personal computers and facsimile machines, etc.
A variety of printing methods used in printers are known in the art. In recent years, however, the ink-jet printing method has become the focus of attention because this method allows contactless printing on print media such as paper, readily lends itself to color printing and is extremely quiet. In terms of the structure thereof, serial printing is in wide use generally owing to low cost and compactness. In serial printing, the apparatus is equipped with a mounted printhead that discharged ink in accordance with desired print information and printing is carried out while the printhead is scanned back and forth in a direction that intersects the direction in which the print medium is fed.
Generally, in a color ink-jet printer, images are often expressed using the three colors of cyan (C), magenta (M) and yellow (Y) or these three colors and the color black (K). Recently, in order to provide even better image quality, there have been proposed a system (a variable-dot ink system), in which amount of ink discharged is varied in such a manner that dots formed by the same ink will differ in size, and a system (color ink system), in which a plurality of inks of the same color system but of different densities are used for the purpose of raising tonality.
A method of discharging ink droplets of different volumes from the same nozzle has been proposed as a method of obtaining different volumes of discharged ink (e.g., see the specification of Japanese Patent No. 3058493). According to this method, it is known to provide two or more types of heaters of different sizes in a case where the ink droplets are discharged utilizing thermal energy, or to control voltage, which is applied to a piezoelectric element, in several stages in a case where the ink droplets are discharged by compressing an ink chamber using the piezoelectric element.
However, in a case where ink droplets of different sizes are discharged from the same nozzle, it is difficult to reduce the size of the nozzle and to control the discharge of ink stably.
It is believed that the above problems can be solved if a nozzle row for discharging large ink droplets and a nozzle row for discharging small ink droplets are provided. In this apparatus, stable control of ink discharge can be achieved because each nozzle only discharges ink droplets of one size.
Furthermore, in an apparatus having a plurality of nozzle rows that thus discharge inks of different volumes, an advantage is that the quality of the printed image is improved.
More specifically, owing to advances made in raising nozzle density and improving definition, minute manufacturing error occurs on a per-nozzle basis in the manufacturing process of the printhead and this causes variations in amount of ink discharged and in discharge direction on a pre-nozzle basis. Stripes and unevenness appear in images printed by such a printhead and a decline in image quality results.
However, in an arrangement equipped with a plurality of nozzle rows that discharge ink droplets of different volumes, the same pixel (dot) can be printed using different nozzle rows in a single scan. This means that even if multiple-pass printing is not carried out, one pixel is printed by a plurality of ink droplets discharged from different nozzles, and therefore the effects of differences in individual nozzles are mitigated, thereby reducing stripes and unevenness and improving image quality.
For the above reasons, ink-jet printers that carry out high-quality printing are believed to be advantageous in terms of the structure having a plurality of nozzle rows that discharge ink droplets of different volumes.
However, adopting an arrangement having a plurality of nozzle rows for discharging ink droplets of different volumes leads to a unique problem, which will now be described.
FIG. 2 is a diagram in which an ink-jet printhead unit is seen from a printed surface, the unit having two nozzle rows per color for discharging ink droplets of different volumes. In the example illustrated, one printhead is provided-with one nozzle row. Starting from the left side in FIG. 2, printheads are disposed in the following order: a printhead 11C for discharging cyan (C) ink droplets of large volume, a printhead 11sc for discharging cyan (sc) ink droplets of small volume, a printhead 11M for discharging magenta (M) ink droplets of large volume, a printhead 11sm for discharging magenta (sm) ink droplets of small volume, a printhead 11Y for discharging yellow (Y) ink droplets of large volume, and a printhead 11sy for discharging yellow (sy) ink droplets of small volume.
Each printhead has 128 nozzles at a pitch of 600 dpi. The printheads 11C, 11M, 11Y discharge ink droplets of approximately 5 ng as ink droplets of large volume (dots of large size), and the printheads 11sc, 11sm, 11sy discharge ink droplets of approximately 2 ng as ink droplets of small volume (dots of small size).
As will be understood from the drawings, a printhead that discharges large dots and a printhead that discharges small dots of the same color are arranged next to each other. The reason for this is that when two printheads using the same color ink are arranged next to each other, an advantage gained is that a common ink tank can be employed for these two printheads, thereby making it possible to simplify the passageway from the ink tank to the printhead.
FIG. 10 is a schematic view illustrating printing in progress as seen from the side. Here only the cyan printheads are shown in order to simplify the description. A printhead unit 5 is provided with the printhead 11C that discharges large dots Cd and the printhead 11sc that discharges small dots scd. The ink droplets are discharged toward printing paper 7 in accordance with an image signal while a carriage is being moved in the direction of the arrow.
Since the ink droplets are discharged while the carriage is moving, the discharged ink droplets have a velocity component in the traveling direction of the carriage. As a consequence, the ink droplets are subjected to the effects of air currents and this has an effect upon ink impact position. In particular, the small dots scd also are affected by turbulence produced by the large dots Cd discharged from the neighboring printhead 11C. This makes it difficult to control the impact position. As a result, a disturbance is produced in the impact position of the small dots, this disturbance in impact position appears as stripes and unevenness in the printed image and causes a decline in the quality of the printed image. The larger the number of large dots discharged, the more pronounced this phenomenon becomes.